git clone writes to sderr, fine but why can't I redirect to stdout

Like many Unix utilities, git-clone will be quieter if it's redirected to a pipe. The assumption is the output is useful to a human and it will just get in the way of a program. tee breaks this assumption, but git can't know what's at the end of the pipe.

From the git-clone manual...

--progress

Progress status is reported on the standard error stream by default when it is attached to a terminal, unless -q is specified. This flag forces progress status even if the standard error stream is not directed to a terminal.

You have to specify git clone --progress to force it to do the full output.